Cooking is normally done by putting food into a vessel that is on a fire. The question arises: If one has a pot that is not on a fire or on a plata, but that contains a liquid that is yad soledet bo or hotter, may one place uncooked food into this hot liquid?
In halakha, only certain types of vessels are considered capable of cooking, while others are not considered capable of cooking. In order to elaborate on this idea, we must first define several terms. A kli rishon is a pot or pan that has been heated on the fire. A kli sheni is a pot or pan into which water or food from a kli rishon has been poured. There is a halakhic difference between a kli rishon and a kli sheni. Since a kli rishon was on the fire, the walls of the pot became heated from the fire directly. Therefore, it retains its heat for a long time, and can cook raw food that is placed in it. In contrast, the walls of a kli sheni were not heated directly by fire, so the water or food placed in it cools down quickly. Thus, a kli sheni is not capable of cooking raw food that is placed in it (Tosafot, Shabbat 40b, s.v. “shema mina”).
However, there are some foods, including certain types of fish, that are unique in that they cook quickly and easily. These foods, known in halakhic literature as kalei ha-bishul (easily cooked), can become cooked even in a kli sheni. Thus, cooking kalei ha-bishul is prohibited by Torah law even in a kli sheni. In addition, even irui (pouring water over them) from a kli sheni is forbidden. Since kalei ha-bishul are so easy to cook, even this can cook them and render them edible (MB 318:36).
In principle, it would seem that only a few foods truly qualify as kalei ha-bishul. Nevertheless, some prominent poskim were concerned that we do not know how to differentiate between kalei ha-bishul and regular foods. According to them, only if we know explicitly that a particular food is not included in kalei ha-bishul, like water or oil, may it then be heated in a kli sheni (MB 318:42; SSK 1:59). Additionally, according to some prominent poskim, it is rabbinically prohibited for food that was not cooked before Shabbat to be placed in a kli sheni on Shabbat, because this resembles cooking (MA; MB 318:34). Therefore, in practice one should not place food that has not already been cooked into a kli sheni, but irui from a kli sheni onto an uncooked food is only prohibited when one knows for certain that a specific food is among kalei ha-bishul.
Therefore, one may prepare an instant soup mix by pouring water from a kli sheni into the dry mix. However, one may not prepare instant mashed potatoes this way, because doing so violates the prohibition of Lash (see 12:7 below).
There is a third type of vessel known as a kli shlishi. If one pours hot water or hot food from the pot in which it was cooked into another vessel, and from that vessel into a third one, that final container is a kli shlishi. The poskim agree that a kli shlishi is unable to cook anything.
. In truth, some Aĥaronim were inclined to be stringent and avoid putting anything raw and easily cooked into a kli shlishi that is yad soledet bo. Thus states Shevitat Ha-Shabbat, Mevashel 23, based on Yere’im. This is also the position of Ĥazon Ish regarding kalei ha-bishul (Ĥazon Ish, OĤ 52:19). He maintains that as long as the water is hot, no matter how many times removed the vessel is from the original kli rishon, kalei ha-bishul become cooked. AHS 318:28 states this specifically with regard to tea. According to Ĥayei Adam 20:4, any vessel whose contents are so hot that they would burn someone is capable of cooking. However, according to most poskim, the principle that Bishul does not apply in a kli shlishi is absolute, and any kind of raw food may be introduced into a kli shlishi. MB 318:47 records this based on Pri Megadim. The accepted explanation is that this was the Sages’ assumption – cooking is inconceivable in a kli shlishi. Further, it seems to me that cooking in a vessel that people do not generally use for cooking would not be prohibited by Torah law, since the Torah prohibition applies only to cooking in the usual manner. Since one normally does not cook in a kli sheni, there is no Torah prohibition of putting raw food into a kli sheni. However, foods that cook easily are often cooked in a kli sheni or even by irui from a kli sheni. Therefore, if one places these foods in a kli sheni or pours water on them from a kli sheni, he transgresses a Torah prohibition. However, not even kalei ha-bishul are generally cooked in a kli shlishi, so there is never a Torah prohibition involved. And since in the vast majority of cases one cannot cook in a kli shlishi, the Sages did not prohibit cooking in one in any case.MA, MB 318:34, and Kaf Ha-ĥayim §70 state that the halakha follows the first opinion presented in Tosafot, Shabbat 39a. This opinion states that even though a kli sheni does not cook, one may not place raw food into such a vessel because it resembles cooking. One may, however, add spices, since that does not resemble cooking. This is also the position of Or Le-Tziyon 2:30:5. In contrast, R. Ovadia Yosef, basing himself on a number of Rishonim and Aĥaronim, writes that the halakha follows the second opinion in Tosafot, according to which there is never a concern of resembling cooking in a kli sheni (Yeĥaveh Da’at 6:22).
Some maintain that since we do not know what foods are considered kalei ha-bishul, we must be stringent and refrain from putting any foods into a kli sheni except those that we know are not kalei bishul (Yere’im; Smag). Others maintain that only specific foods that are known to be kalei ha-bishul are a concern (Ran; Tur). Rema 318:5 states that the custom is to be stringent, as do MA 318:18; SAH 318:12; Ĥayei Adam 20:4; MB 318:42; SSK 1:59. SA 318:5 cites both opinions and seems inclined to be lenient. This was the inclination of a number of poskim – that one need be stringent only with foods that are known to cook easily (Ĥazon Ish, OĤ 52:18; Or Le-Tziyon 2:30:3). Yalkut Yosef 318:47 also records this as the position of Rambam and Maharam ibn Ĥabib.
To simplify the matter, I wrote to be consistently stringent in the case of a kli sheni, and consistently lenient in the case of a kli shlishi. Even though it is agreed that one may not pour from a kli sheni onto kalei ha-bishul, nevertheless we have seen that according to most poskim, most foods are not kalei ha-bishul. Moreover, even those who are stringent consider the prohibition rabbinic, since one does not intend to cook. Additionally, pouring will only cook the outer layer of the food, which is less than the amount required to transgress a Torah prohibition, and according to Rashbam this is not considered cooking at all. Therefore, one should only be stringent and refrain from pouring from a kli sheni in the case of foods that are known to be kalei ha-bishul.